One of the principles guiding the design of the 5th generation mobile networks (5G) and New Radio (NR) system is the transparency of the network to an user equipment (UE). As such, the UE is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments. For example, antenna ports belonging to different transmission points can transmit different downlink control information (DCI) messages on a physical downlink control channel (PDCCH). While there are various reasons for different transmission points transmitting control signaling to a UE, one application relates to distributing parts of the scheduling algorithm to different transmission points so that downlink transmissions are associated with a different transmission point than uplink transmissions. In this case, it makes sense to schedule downlink and uplink transmissions with control signaling provided directly from the respective transmission points.
In another application, different transmission points simultaneously transmit data to a UE to increase the data rate or to support handover between transmission points. In yet another application, a serving transmission point transmits control information to a UE while other transmission points (e.g., pico nodes) transmit data to the UE. In these applications, different transmission points may transmit control signaling such as on a PDCCH in the same subframe to the UE. Further, for each application, UEs might not be aware of the geographical location of each antenna port.
A demodulation reference signal (DMRS) or a UE-specific reference signal (RS) is employed for demodulation of data channels and possibly certain control channels (e.g., PDCCH). A UE-specific RS relieves the UE from having to know many of the properties of the transmission and thus, allows flexible transmission schemes to be used from the network side. This is referred to as transmission transparency (with respect to the UE). However, one problem is that the estimation accuracy of a UE-specific RS (i.e., channel properties) may not be good enough in some situations.
Geographical separation of RS ports implies that instantaneous channel coefficients from each port towards the UE are in general different. Furthermore, even the statistical properties of the channels for different antenna ports and RS types may be significantly different. Examples of such statistical properties include the received power for each antenna port, the delay spread, the Doppler spread, the received timing (i.e., the timing of the first significant channel tap), the number of significant channel taps, and the frequency shift. In LTE, nothing may be assumed about the properties of the channel corresponding to an antenna port based on the properties of the channel of another antenna port. This is in fact a key part of maintaining transmission transparency. Based on the above observations, the UE needs to perform independent estimation for each antenna port of interest for each transmission. This results in occasionally inadequate channel estimation quality for certain antenna ports, leading to undesirable link and system performance degradation.
In LTE and NR, reference signals used for channel estimation may be associated with respective antenna ports. In one example, over one antenna port, a reference signal or set of references signals may be transmitted. From different antenna ports, different reference signals or different sets of reference signals may be sent so that an antenna port may be distinguished from another antenna port by means of the transmitted reference signals or sets of reference signals. Hence, the UE may estimate the channel from one antenna port by using the associated reference signal. Further, a certain data or control transmission is associated with an antenna port, allowing the UE to use the reference signal for that antenna port to estimate the channel used to demodulate the associated control or data channel. Also, the data or control channel is transmitted using that antenna port.
Furthermore, simultaneous uplink data transmissions by many UEs to desired transmission points, e.g. base stations, in a wireless communication system are typically not only received by the desired base stations but are also received in the form of interference by other base stations. Such uplink interference exacerbates uplink precoded transmissions based on channel reciprocity. In particular, interference for UEs that select the uplink precoder (e.g., non-codebook-based precoding approach) should be kept low since the chosen precoder may not be under network control. Accordingly, there is a need for improved techniques for reducing interference in a wireless communication system. In addition, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and embodiments, taken in conjunction with the accompanying figures and the foregoing technical field and background.
The Background section of this document is provided to place embodiments of the present disclosure in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.